DESCRIPTION: (Applicant's Description) There is increasing evidence that phytoestrogens, also referred to as isoflavones, which are derived from soy, in addition to possessing antiproliferative properties, may alter the plasma concentration, production, metabolism and excretion of estrogens and their impact on target tissues. Definitive prospective studies testing the effects of specific components of isoflavones that alter biomarkers that are implicated in the initiation and promotion of breast cancer are not available. The investigators propose a controlled, randomized clinical trial where 66 pre menopausal, omnivorous, breast cancer free women between ages 25-55 are randomized to one experimental group supplemented with soy (40 mgs of genistein/day) or to a control group consuming a placebo, an isocaloric supplement of casein, for a 12 week period and wish to observe changes in anthropometric, nutritional and hormonal biomarkers. They hypothesize that increased intake of genistein will produce an elevation in serum sex-hormone binding globulin and decrease a free estradiol and estrone. In their pilot study, they were able to demonstrate the feasibility of their proposal to maintain a group of women on soy supplementation without altering intake of other nutrients. In previous studies, they have demonstrated their ability to monitor changes in anthropometric and sex-hormone variables in similar populations and in breast cancer patients. If increased intake of soy or foods rich in phytoestrogens alter the sex hormone milieu, manipulation of diet, supplemented with foods rich in phytoestrogens such as soy intake by dietary intervention may reduce breast cancer risk. In addition, based on the results of this study, prophylactic therapies using dietary supplements such as genistein, with practically no side effects, may be used to replace the more controversial therapeutic hormonal supplementation regimens that are currently used for breast cancer risk reduction. In addition, studies examining the effect of genistein supplementation, in addition to fat reduction on both body composition, weight and sex-hormones can be examined with respect to hormonal cancer risk reduction. Although conducted on a female population, based on evidence in the current literature, it is anticipated that this proposed study will generate results which parallel effects in prostate cancer risk reduction.